Controlled transmission circuit



Feb. 24, 1942. K, SINGER I y GOTROLLED TRANSMISSION CIRCUIT '2' shets-sheet 1 FiledA Dec. y1, 1939 Wmv n n 4 Y I B f Patented Feb. 24, 1,942

2,274,505 CNTROLLED TRANSMISSIQN CIRCUIT Kurt Singer, Burbank, Calif., assigner to Radio Corporation of America, a corporation of Del.-

6 Claims.

This invention relates to sound transmission circuits and particularlyto a sound reproduction system wherein a primary signal is controlled by a secondary or control signal in a predetermined manner.

The transmission or gain of sound reproducing systems, in which the sound originates from sources such as film sound tracks, disc records and microphones, has been varied during operation in different manners, variations of certain types being known as compressions and expansions. It is well known in the art how to provide nonlinear gain characteristics foran amplifier, a linear characteristic being one wherein the amplitudes of the output signal vary in the same ratio as theamplitudes of the input signal. One common method of obtaining compression 'andexpansion is-to impress upon theA l grids of variable mu tubes a bias voltage which varieslinearly with the transmitted signal amplitude, and, since the gain of the tubes is exponential with grid bias, the resulting char-acteristic between input and output is nonlinear. Although the amplifier gain is usually controlled by the signal being transmitted, it may .also be controlled by an external or extraneous signal in a manner such as disclosed and claimed in Mueller Patent 1,888,467 of November 22, 1932. In the past, where a primary signal has been controlled by a secondary signal, the relationship 2 between the two signals throughout their op erating range has been relatively unimportant as long as the general overall result was accomplished.

The present invention is directed to a control system in which not only is the primary signal lfnder control of a secondary signal, but a defi# nite predetermined relationship is provided between the variations of one signal with respect to the other throughout their entire operating range in a novel manner. The relationship desired is that the variations in the signal being controlled be directly proportional to the variations of the control signal. This relationship is not obtainable by the normal application of the.

rectified control signal to the grid bias of the amplifier tube or tubes, inasmuch as. the gain ofi the amplifier tube or tubes varies exponentially with respect to changes in Ibiasing potential. In the present invention, therefore, not only is the gain of the primary amplifier varied 'by a portion of the rectified secondary or controlling signal, but the controlling signal also simultaneously varies the gain of its own am` plifier.' Thus, there is provided by the control- Application-December 1, 1939, Serial No. 307,089

ling amplifier circuit a correction which permits a biasing voltage to be applied to the primary amplifier in a mannersuch that the output of the primary amplifier varies indirect proportion to the variations in amplitude of the controlling signal throughout its entire range.

The principalobject of the invention, therefore, is to improve the control of a variable gain amplifier.

Another object of the invention is to control the gain of an amplifier in a certain predetermined manner.

A further object of the invention is to obtaina predetermined characteristic of a variable gain amplifier with respect to the variations of a controlling signal.

A further object of the invention is to simultaneously control the gain of an amplifier of a controlling signal which is controlling the gain of an amplifier of a controlled signal.

A further object of the invention is to obtain a linear gain variation in an exponential amplifier.

A further object of the invention is to obtain. a linear relationship between the variations in amplitude of a secondary signalsource and the variations in amplitude of a primary signal source controlled by the secondary si-gnal source.

Although the novel features which are believed to be characteritsic of this invention will be pointed out with particularity in the claims appendedherewith, the invention itself, its objects and advantages, the manner of its organization and the mode of its operation will be better understood by referring to the following description read in conjunction with the accompanying drawings forming a part thereof, in which Figure 1 is a schematic drawing of a sound reproducing system embodying the invention;

Figure 2 is a graph showing a characteristic curve of the amplifying tubes used inthe invention;

Figure 3 is a graph showing the relationship between the input voltage to the controllin-g'a'mplifier and the direct current biasing voltage output of the controlling amplifier-rectifier; and

Figure 4 is a graph,showing the relationship between the input level to the controlling amplifier and the gain of the .controlled amplifier.

Referring now to Fig. 1, one form of primary or controlled signal source is diagrammatically represented as a sound film 5- supplied from a reel B and taken up by a reel l and which is scanned by a light'beam produced'by a source of light 8 impinging upon a photoelectric cell 9 the usual optical system and film drive being employed but not shown. The output of the photocell 9 is impressed upon a standard linear amplifier II and fed into a push-pull amplifier composed of tubes I3 and I4 of the exponential or variable mu type. The amplifier II is coupled to the tubes I3 and I4 by means of a pushpull transformer I6 having loading or compensating resistances I1 across the secondary thereof. Tubes i3 and I4 are connected by condensers I9 and 28 and resistances 2l and 22 to the input of a. push-pull standard linear amplifier 24 and 25, the output of which is transmitted lby a transformer 26-to a linear amplifier 21 which feeds a loudspeaker 28. Potential for the anodes of the tubes 24 and 25 is supplied over conductors 38 and 3| connected to the plus-B supply as shown. Potential for the anodes of tubes I3 and I4 is supplied over conductor 3I and conductor 33 through resistancel 34. Resistance 34 and resistances 35 form a potential divider from which is supplied the positive potential for the screen grids of tubes I3 and I4 over conductor 31. Bypass condensers 38 are used acrossresistances 34 and 35.

The controlling or secondary signal source may also be in the form of a sound track on a film 48 fed from a reel 3 to a reel 4 and reproduced through any well-known type of system employing a light source 4| and a photoelectric cell 42. The output of cell 42 is impressed upon a linear amplifier and bandpass filter unit 43. The bandpass filter is only necessary when the control Signal is one of a plurality of frequencies recorded in the same track area with control signals of other frequencies. The output of unit 43 is impressed upon an exponential vacuum tube 45, similar totubes I3 and I4, through an input transformer 44 which is loaded on the primary side by a resistance 41. 1The output circuit of the tube 45 is coupled through a condenser-resistance network 48 to a second exponential ampliiier tube 46, the output circuit of this tube being also coupled through a condenser-resistance network 49 to a triode vacuum tube 58. The output of tube 58 is transmitted through a three-winding transformer 52 having secondary windings 53 and 54. The anode potential source for the tubes 45, 46 and 58 -is the same as for the tubes in the primary ampliiier, tubes 46 and 50 being connected to the plus-B terminal over conductors 56 and 51, through resistances 59 and 68,.respectively, and tube 45 through resistance 6I. The screen grids of tubes 45 and 46 are fed through potential divider resistances 64 and 65, respectively, having bypass condensers 66 and 61.

The output of the secondary or controlling amplier just described is impressed upon two rectifiers 18 and 1I, rectifier 1| controlling the gain of ampliiier tubes 45 and 46, while rectier 18 controls the gain of amplier tubes I3 and I4. The output from tube 58 is impressed upon rectifier tube 18 over the secondary winding 53 of transformer 52 and conductors 13 and 14. The output of tube 58 is impressed upon rectifier 1I over the secondary winding 54 and conductors 16 and 11.

As mentioned above, the tubes 45 and 46 obtain their grid biasing potential, in addition to the bias obtained through self-biasing resistances 80 and 8| shuntedby condensers 62 and 83, respectively, from rectifier 1I which is biased positively by resistances 85 and 86. The biasing circuit for tube 45 is from the grid of this tube through the secondary of transformer 44. iilter resistance 88, conductor 16, control biasing resistance 89, shunted by bypass condenser 98, conductor 9|, to the cathode of tube 45 through selfbiasing resistance 88. The grid bias for tube 46 is obtained over a circuit'from the grid of this tube through coupling resistance 93, over conductor 16, through resistance 89, over conduce tors 9| and 92, and self-biasing resistance 8|'. Thus, the output of tube 58 will vary the gain of tubes 45 and 46 in accordance with the output from the rectiier 1I over the above-traced circuits and, consequently, the voltage in secondary winding 53 feeding rectier 18 will be varied in' accordance with the characteristic of the control of the tubes 45 and 46.

The grid bias for tubes I3 and I4 in the pri-Y mary amplier is controlled by the output of rectiiier 18 which opposes a xed bias obtained from apotential source 94. The grid bias circuit for tubes I3 and I4 is from the grids of these tubes, through the secondary windings and loading resistances I1, through iixed bias potential source 94, conductor 95, to adjustable contact 96 on resistance 91, this resistance and' rectifier biasing resistance 98 being shunted by bypass condenser 99. The remainder of this circuit is through the resistances 91 and 98,V over conductor I 88, to thev -plier circuit including tubes I3 and I4 is theA finally controlled amplifier transmitting the primary signal from the lm 5 for reproduction in loudspeaker 28. The ampliiler circuit including tubes 45 and 46 is the controlling amplifier circuit transmitting the controlling signal from the film 48. 'I'he output of the controlling amplifier is divided, one portion being utilized to control the gain of the controlling ampliiier, and the other portion being used to control the gain of the controlled amplifier. By this combination, a linear relationship between the amplitude variations of the controlling signal and the gain variations of the controlled-amplier is obtained.

To further explain the operation of this circuit, referenee is made to Figs. 2, 3 and 4, in which Fig. 2 illustrates a characteristic curve for typical exponential tubes shown at I3, I4, 45 and 46. It will be noted from this characteristic that the curve is exponential, that is, as the control grid voltage increases in a positive direction, the mutual conductance of the tube, which represents its amplification or gain, increases as an exponential function. Thus, if the tubes I3 and I4 were impressed with a grid biasing potential varying linearly with the variations in the controlling signal, the resulting output 0f tubes I3 and I4 would not be linear with respect to the amplitudes of the controlling signal. An exf ponential systembf this type is disclosed and -claimed in Kreuzer Patent 2,006,052 of June 25,

By referring to Fig. 3, the relationship between the input voltage to the controlling amplifier and 4 This characteristic is the one'.desired and is provided inasmuch as the grid biasing voltage output from rectifier has -a characteristic as shown by curve b in Fig. 3. If the linear relationship shown by the characteristic a of Fig. 3 existed between the controlling signal and the direct current biasing voltage provided thereby, which is the normal relationship, then the characteristic between the input controlling level and the gain of the controlled amplifier would be as shownl by 'curve d.

In addition to the linear characteristic c, it is Y also possible with the invention to produce other types of linear characteristics having diierent amplification ratios such as shown by dotted lines le and f. Although the ordinate in Fig. 4 is gain in db. ofthe controlled amplier, this ordinate may also represent the output o'f lthe controlled device, thus showing a linear relationship between the input voltage of the controlling signal and the output voltage of the controlled signal provided the input amplitude of the control signal 'is constant.

It is to be understood that although the specic arrangement described above provides for minimum gain of the controlled amplier with minimum controllingsignal, the invention alsoncontemplates an arrangement providing maximum gain o f the controlled amplifier with minimum to the output of said second ampliiier, the output of one of said rectifiers being connected to said first-mentioned amplier, and the output of another of said rectiers being connected to said second-mentioned amplier.

3. An electrical transmission circuit comprising a variable gain amplifier for transmitting a signal to be controlled; a second variabie gain ampliiier for transmitting a controlling signal for said first-mentioned amplifier, and means interconnecting the output circuit of said second l amplier and the gain control circuit of said mst-mentioned amplier for producing a linear relationship between thevariations in amplitude of said controlling signal and the variations in gain of said controlled amplifier.

4. An electrical transmission circuit comprising a variable gain amplifier for transmitting a signal to be controlled, a second variable gain amplifier for transmitting a controlling signal for said rst-mentioned amplifier, and means interconnecting the output circuit of said second amplifier and the gain control circuit of said first-mentioned amplier for producing a linear relationship between the variations in amplitude of said controlling signal and the variations in gain of said controlled amplier, said last'- mentioned means comprising a pair of rectifiers,

amplitude of controlling signal. This is accomplished by reducing the fixed 'bias potential source 94 to a suitable lesser value and reversing the output of rectier 10. It is also to be understood that rectiiers 'I0 and 1| may be fullwave rectiers instead of half-wave, as shown. What I claim as my invention is:

1. An electrical transmission circuit comprising a variable gain amplifier for transmitting a signal to be controlled, a second variable gain amplier for transmitting a controlling signal, a plurality of rectiiiers; means interconnecting -the output of said second amplifier with one of said rectiers for controlling the amplitude of the output of said second amplifier in accordance with the input amplitude to said second ampli- Iier, and means for connecting the output of said second amplier with another of said rectiflers for controlling the amplitude of the output of said rst-mentionedamplier in accordance with the controlled output of said second ampliiier.

one'of which controls the gain'of said first-mentioned amplifier, and the other of which controls the gain of said secondmentioned amplier.

5.,An electrical transmission circuit compris- `ing an amplication circuitlor a primary signal adapted to be controlled, an amplification circuit for a. secondary signal adapted to vary in amplitude for controlling said primary signal,

means for controlling the ratio of ampliiication of said secondary signal by said secondary signal, and means for controlling the ratio of ampliiication of said primary signal by the controlled secondary signal, the ratios of ampliiication pro- Y viding a linear relationship between the variations in amplitude of said secondary signal and the variations in gain of said amplifier circuit for said primary signal. l

6. A system of signal transmission control comprising a source of controlling signal currents varying in amplitude, a source of signal currents to be controlled, means for transmitting said controlled signal currents, said means including an amplier having non-linear gain variations with respect to variations in biasing voltage',

means for translating said controlling signal cur- 2. An electrical transmission circuit'comprising a variable gainamplier forv transmitting a' rents into a biasing voltage adaptedto be impressed on said amplier, and means included in said last-mentioned means. forvarying the translation't the variations in amplitude of 4said controlling signal to variationsin biasing voltage in' a ratio inversely proportional to the gain variation` characteristic of the` controlled amplifier.

KURT SINGER. 

